also: かっぱ
noun
A waterproof coat worn in the rain; from Portuguese 'capa'.
from Portuguese capa
雨が降りそうだから、カッパを持って行こう。
It looks like rain, so let's take a raincoat.
子供の頃、黄色いカッパを着て学校に
When I was a child, I wore a yellow raincoat to school.
レインコート is a more modern, general term for raincoat, while カッパ often implies a traditional or simple waterproof coat, sometimes associated with children's wear.
From Portuguese 'capa' (cloak, cape). The ateji spelling 合羽 is a phonetic rendering.